This application claims the priority of Korean Patent Application No. 2006-0066218, filed on Jul. 14, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Technical Field
The present disclosure relates to a data modulator, and more particularly, to a data modulator based on Gaussian minimum shift keying (GMSK) modulation and a data transmitter including the same.
2. Discussion of Related Art
GMSK is a kind of minimum shift keying (MSK) and is a modulation scheme by which rapid change in binary data is removed by smoothing phase changes using a Gaussian pulse shaping filter, so that the bandwidth of the modulated signal is reduced.
FIG. 1 illustrates the impulse response of a Gaussian pulse shaping filter with respect to bandwidth (BT). Referring to FIG. 1, the impulse response of the Gaussian pulse shaping filter that exists in a range from t=−∞ t=+∞ causes inter-symbol interference (ISI). When BT decrease, symbol energy a α, that is, the power corresponding to α, increases and, thus, ISI also increases.
FIG. 2 illustrates the power spectral density (PSD) of MSK and GMSK. Referring to FIG. 2, the power spectral density (PSD) of GMSK has lower side-lobe power and narrower main-lobe width than that of MSK. When filtering is performed using a baseband Gaussian filter before MSK modulation, side-lobe power and main-lobe width can be reduced. In this case, the slope of the main lobe increases more and the level of the side lobe decreases more. This kind of MSK modulation with Gaussian filtering of a baseband signal is referred to as GMSK modulation.
In GMSK modulation, a main-lobe width and a side-lobe level are determined by BT. BT is a standardized bandwidth and is obtained by multiplying a 3-dB bandwidth B of a Gaussian filter by a symbol interval T. When the BT is decreased, the main-lobe width and the side-lobe level are also decreased. Accordingly, when the BT is small, the adjacent channel interference (ACI) is also small.
Referring to FIGS. 1 and 2, ISI and ACI are in a trade-off relation.
GMSK is usually used in communication systems in which a narrow band width is allocated to a single user. A global system for mobile communication (GSM) is one of these communication systems. The GSM uses GMSK modulation in which BT is 0.3. This means that the 3-dB bandwidth of a Gaussian filter used for GMSK modulation is 0.3*271 kHz. The value of BT is obtained by performing a trade-off analysis of ISI and ACI and selecting a value giving optimal performance.
FIG. 3 is a block diagram of a conventional data modulator 300 based on GMSK modulation. Referring to FIG. 3, the data modulator 300 includes a GMSK modulator 310, digital-to-analog converters 320 and 325, and low-pass filters 330 and 335. The GMSK modulator 310 includes a shift register 311, a counter 312, an I-channel read-only memory (ROM) table shown at 313, a Q-channel ROM table shown at 314, and latches 315 and 316.
In GMSK modulation, input digital information is Gaussian filtered bit by bit integrated value are output as I-channel data and Q-channel data, respectively. When this operation is actually performed in hardware, however, computation is very complicated and power efficiency is not good. For this reason, the GMSK modulator 310 usually has the I-channel ROM table 313, in which results of performing GMSK modulation on an I-channel are stored in advance, and the Q-channel ROM table 314, in which results of performing GMSK modulation on a Q-channel are also stored in advance.
The impulse response of a Gaussian pulse shaping filter used in GMSK exists in a range from t=−∞ to t=+∞, but when |t| increases from a reference point, t=0, the impulse response becomes so small as to be negligible. Accordingly, when a GMSK modulator is implemented in the form of a look-up table, it is assumed that the magnitude of an impulse response is 0 when |t|>nT where “n” is a real number. For example, when n=1.5, a GMSK modulation value for a current bit is calculated considering both a previous bit and a subsequent bit of the current bit.
The shift register 311 shifts an input digital information signal Inf, which is input in a bitstream, by one bit and outputs a data signal corresponding to the number of bits, which should be considered, for example, 3 bits when n=1.5, in parallel. The I-channel ROM table 313 and the Q-channel ROM table 314 output GMSK modulation values corresponding to the data signal received from the shift register 311. Since a single GMSK modulation value includes a plurality of samples, the counter 312 outputs a count signal for sequentially outputting the plurality of samples to the I-channel ROM table 313 and the Q-channel ROM table 314. The latch 315 latches the GMSK modulation signal output from the I-channel ROM table 313 and the latch 316 latches the GMSK modulation signal output from the Q-channel ROM table 314.
Each of the latched GMSK modulation signals is converted into an analog signal by one of the digital-to-analog converters 320 and 325, processed by on of the low-pass filters 330 and 335, and transmitted to an I-channel or a Q-channel, respectively.
GMSK modulation values stored in the I-channel ROM table 313 and those stored in the Q-channel ROM table 314 are calculated based on a fixed BT value, for example, BT=0.3, for GMSK modulation in a GSM. Even if an optimal BT value is selected considering ISI and ACI, the channel state may vary according to the external environment and, therefore, a method of adaptively controlling ISI and ACI according to the channel state is desired.